Christmas lights and other RFI---Bah humbug

Just kidding, I am in the holiday spirit, but I just got off the phone with Stuart Lipoff, IEEE Fellow and we discussed the possibility of RFI from Christmas lights and other sources. Lipoff says that how you're affected really comes down to what type of Christmas lights you use. The older technology that was used to make the lights blink can indeed cause radio interference. In this case, the lights were arranged into a string of small low voltage incandescent lamps in series with each other. The total number of lamps in the string were such that the combined voltage of all of them was equal to the 110 volts in a typical US home.

Most modern day Christmas tree lights are based on solid-state LEDs and often use an external electronic flashing controller and do not create radio noise. However, there are some LEDs that have an additional blink controlled chip right inside the LED bulb. It turns out that these devices also create significant radio interference as this internal controller cycles the LED chip from on to off. The reason is somewhat analogous to the spark problem in incandescent blinker bulbs in that when the LED is between fully on and fully off it can exhibit negative resistance that causes it create radio energy.

Lipoff told me that the UK regulator OFCOM claims that Christmas tree lights can slow down Wi-Fi speeds. The interference is usually in the lower frequency ranges which would affect more the AM Radio band (530 kHz to 1.6 MHz) and not so much the FM radio band which starts at 88 MHz. He says that some people take a battery-operated AM radio and walk around the house, with the radio tuned between stations, and the RFI can he easily heard as you come near an offending device whether it’s Christmas lights, a wall light dimmer (Ones by GE or other brand name manufacturers are usually good but the ones from the $1 store are typically noisy), hair dryer or essentially any device that has a motor that will create sparks from the brush/armature connection as the motor spins.

This phenomena is what helped send Morse Code from Ships at sea due to this negative resistance effect sent out over an antenna. Here’s how the ARRL describes it.

Wi-Fi is another story. 802.11b is up at 2400 MHz so will not be as affected as an AM radio. Lipoff says that Wi-Fi will re-send something that is not received properly anyway.

Latest low voltage switching power supplies for lights can be noisy in the lower RF ranges as well. These are used for PC power supplies as well but they must adhere to FCC rules because they are classified as a computer device.

How about a tropical fish tank heater? They usually have no hysteresis and use a bi-metal strip as a thermostat which will arc.

I am putting all of this out there starting the holiday season in order to get your experiences with noise that interferes with modern equipment like a Wi-Fi system or other device. Please do send your experiences and share them with our audience.

The whole topic is interesting as we transition to all LED lighting, eventually. I still prefer the warmth of incandescents, tbough LEDs are getting better. Or, maybe I am just getting used to them and starting to like not having to change 20 bulbs every year (or throw out a full set of lights again, like I did this Christmas).

To detect EMI, grab a handheld spectrum analyzer (I'm with R&S so I naturally have an affinity toward R&S equipment, such as the Spectrum Rider ZPH.

Connect a directional antenna and you can quickly find the offending string of lights. And since it can measure signals up to 4 GHz, you can see if your lights—or those of your neighbors—are really slowing down your Wi-Fi:)

Switched mode power suppliers are often at the heart of our EMI and RFI issues during certification, but other simple devices and circuits also create a lot of wideband RFI.

I worked on a circuit where the conmutation of one small relay at the same time of a few LEDs produced a wideband perturbation in the VCC lines. This perturbation was coupled to a pair of external cables and produced RFI in all bands from 30MHz to 2.5GHz with peaks spaced only a few MHz.

Incandescent bulb christmas lights are still out there, some of them produce a lot RFI when dimming, at some times enough RFI to interfere with the TV nearby the christmas tree.